mtd: nand: pxa3xx: Split FIFO size from to-be-read FIFO count

Introduce a fifo_size field to represent the size of the controller's
FIFO buffer, and use it to distinguish that size from the amount
of data bytes to be read from the FIFO.

This is important to support devices with pages larger than the
controller's internal FIFO, that need to read the pages in FIFO-sized
chunks.

In particular, the current code is at least confusing, for it mixes
all the different sizes involved: FIFO size, page size and data size.

This commit starts the cleaning by removing the info->page_size field
that is not currently used. The host->page_size field should also
be removed and use always mtd->writesize instead. Follow up commits
will clean this up.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>